The research is clear: At around age 40, women start working harder to rediscover old friends — and to make new ones. It’s as if a voice in their heads starts whispering: “You need women in your life!”

I learned this while spending two years immersed in the friendship of 10 women, now in their mid-40s, who grew up together in Ames, Iowa. I was writing “The Girls from Ames,” a book tracing their friendship, and I found that their lives mirrored the results of a host of studies.

When women are in their twenties and thirties, they have to work harder to stay connected because those are the years when women are starting their careers, getting married, having babies. They’re busy. But at around age forty, they often start yearning again for old friends.

In studies before the 1990s, researchers attributed this uptick to women’s lockstep march through the life cycle. After a couple of decades spent finding a mate, building a marriage and raising kids, women finally had time for themselves because their kids were more self-sufficient. In previous generations, at age 40, the average woman already had sent her oldest child off to college or into the workforce, while her youngest child likely was in high school.

These days, at age 40, a woman might be busy having her first child or starting her second marriage. (Indeed, when the Ames girls hit 40, none of them had children older than age 13. One of them, Jenny, still hadn’t had her first child.) Yet, in this new century, even women busy with careers and child-rearing duties become more friendship-focused entering their forties.

“We’ve begun to understand that it has to do with a life stage,” one researcher told me. “In their early forties, women are asking, ‘Where do I want to go with my life?’ Female friends show us a mirror of ourselves. Even lesbians say they see a need for non-sexual relationships with women at about age forty.”

Friends such as the Ames girls, who’ve traveled the timeline together, tend to have more empathy for each other’s needs and ailments. They knew each other when they were younger and stronger, and they’ve watched their bodies change. Studies show that having a close group of friends helps people sleep better, improve their immune systems, stave off dementia, and actually live longer. In fact, researchers say a woman who wants to be healthier and more psychologically fit in her old age is better off having one close friend than half-a-dozen grandchildren.

The Ames girls haven’t tracked all of this research. They just feel it in their guts.

Cathy, a makeup artist in Los Angeles, says she is buoyed by her relationship with her old friends from Ames. “What keeps me going back to them?” she asks. “What is it I don’t want to sever? I think it’s this: We root each other to the core of who we are, rather than what defines us as adults – by careers or spouses or kids. There’s a young girl in each of us who is still full of life. When we’re together, I try to remember that.”

Jeffrey Zaslow is a Wall Street Journal columnist and coauthor, with Randy Pausch, of The Last Lecture, the #1 New York Times bestseller now translated into 41 languages. Zaslow attended Dr. Pausch’s famous lecture and wrote the story that sparked worldwide interest in it. The Girls From Ames also grew out of one of his columns. Zaslow is also the coauthor of the New York Times bestseller Highest Duty by Chesley “Sully” Sullenberger. Zaslow lives in suburban Detroit with his wife and three daughters.

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